The present invention relates to a method for optically measuring or scanning an edge located within or limited by a surface region. To accomplish this, a camera can be used for recording the surface region of interest, said region comprising at least two partial regions that are contiguous to the edge. Subsequently, the edge location can be ascertained based on one or several recordings using at least one edge location criterion for the detection of the position of the edge.
A method for edge measuring has been known, for example, from publication DE 102 15 135 A1. This method is to be used for the automatic detection of control information for values influencing the camera image in order to be able to achieve an optimal measuring result. The information provided by the camera image of a workpiece to be measured can be used to yield auxiliary parameters and, in turn, these auxiliary parameters can be used for deriving control information. Firstly, the characteristics of at least two auxiliary parameters are determined as a function of one or more influence values. For example, contrast, homogeneity, brightness and/or the ascending slope can be used, as auxiliary parameters. In doing so, the characteristics are determined in such a manner that all characteristics exhibit a maximum or a minimum at the optimum. Subsequently, a weighted summation of the individual characteristics of the auxiliary parameters relative to the total characteristic line is determined. The weighting factors can be experimentally determined, for example. Finally, the size of the influence value is determined as the control information for the influence value at the location of the extremum of the total characteristic line. In this manner, it is possible to optimize the measuring setting for recording the camera image and, consequently, for detecting the edge. For edge detection, it is also possible to add several different weighted edge location criteria, for example, a threshold value criterion and a differential criterion, in order to improve the detection of the location of the edge.
Furthermore, a transmitted-light method for the measurement of tools has been known from publication DE 102 37 426 A1. In this case, the capture characteristic of the camera is controlled as a function of a control criterion with the aim of optimizing the capture characteristic in order to be able to exactly determine the contour of the tool in transmitted light. For example, exposure time, signal amplification of the image processing sensor and/or the sensor offset are varied. The control criteria are derived from a comparison with the nominal gray value profiles for the tool contour.
Publication DE 10 2007 003 060 A1 suggests the determination of the quality of a measuring point in edge detection. The pixel values are evaluated with the use of a quality function based on the characteristic of the gray values along measuring lines. The gray value characteristic is weighted by way of the quality and used for the calculation of the edge location.
Publication DE 10 2004 033 526 A1 describes a method and a device for the analysis of at least partially reflecting surfaces. This method concentrates on the examination of the surface and shape of the measured object by using the movement of components of the arrangement. In doing so, use is made of the effect that a pattern reflected by the surface of the object changes if the relative position or orientation of the camera relative to the object surface is changed. This pattern change can be analyzed by a control unit.
In addition publication U.S. Pat. No. 4,912,336 describes a device that is used for the detection of the shape of the surface. In this case, punctiform light sources are sequentially activated and an image of the light reflected by the surface of the object is recorded. The device is intended to be suitable for the varying reflection properties of the surface. The method is to be suitable for surfaces exhibiting specular reflection and also for surfaces exhibiting diffuse reflection properties, as well as for hybrid surfaces. With an extraction algorithm, the components of the specular reflection are separated from the components of the diffuse reflection for each measured image intensity in order to be able to calculate the surface orientation therefrom.
Until now, the exact detection of the location of an edge in the camera image has been difficult, despite various optimizations, for example, the consideration of the control parameters. For this reason, filters are also frequently applied to the recorded images in the course of image processing in order to improve the result while optically scanning the edge.
Considering the cited prior art, it may be viewed as an object of the present invention to provide a highly simple method allowing a detection of the location of an edge by optical scanning with subpixel accuracy.